The present invention is related to the field of vibration and noise control, and in particular is designed for dissipating low frequency vibration and acoustic energy within the payload fairing of launch vehicles.
Current developments in the use of advanced composite structures for fairing design and fabrication has necessitated the development of innovative vibro-acoustic technologies to mitigate the interior acoustic response of launch vehicles during powered flight. Advanced composite fairings provide increased structural stiffness with significantly less mass. As a result, low frequency noise more easily passes through the fairing to the payload area. This excites cavity resonances in the fairing, which may induce vibrations that damage the payload.
There has been considerable research showing that local vibration suppression can be used to reduce sound radiated from a vibrating structure. Johnson, M. E., and Elliott, S. J., Active Control of Sound Radiation from Vibrating Surfaces Using Arrays of Discrete Actuators, Journal of Sound and Vibration, Vol. 207, No. 5, pp. 743-759.) Radiated sound pressure from flexible, vibrating structures can be reduced using either active or passive approaches. This is achieved by using active or passive actuators to change the structural impedance as seen by disturbance inputs, thereby reducing the transmission of energy. In addition, the actuators change the radiation characteristics of the structure, making it less efficient at radiating acoustic energy.
There has also been significant work that demonstrates the use of acoustic resonators as acoustic dampers to dissipate acoustic energy in enclosures. (Doria, A., Control of Acoustic Vibrations of an Enclosure by Means of Multiple Resonators, Journal of Sound and Vibration, Vol. 181, No. 4, pp. 673-685.) These devices are tuned to one or multiple acoustic resonances of an acoustic enclosure. The acoustic energy couples to the device and creates motion of a diaphragm or air mass. The kinetic energy of this motion is dissipated through frictional forces, providing the mechanism for reducing energy in the system.
In order to develop a mechanism to reduce the fairing vibro-acoustic environment, there are several critical engineering design obstacles to overcome. Primarily, the mechanism for attenuation must not interfere with the operation of payload deployment. This requirement prohibits the use of active control approaches that require wire and cable strung throughout the fairing. Preferably, the device would be easy to integrate into the fairing structure and be independent or stand-alone.
The next consideration is the volume-velocity and stroke requirements necessary to significantly affect the high sound pressure level environment that the device will experience during powered flight. At low frequencies (i.e., below 200 Hz), the overall sound pressure level can easily exceed 130 dB. Acoustic energy is predominately concentrated in the frequencies corresponding to fundamental structural resonances and acoustic resonances. It is at these frequencies that control is necessary, since current passive blanket treatments provide little abatement at low frequency. For low frequency attenuation, the stroke requirement (volumetric displacement) will be greater than would be the case for middle and high frequency applications. Volumetric requirements are a key limitation in active structural acoustic control approaches incorporating piezoelectric materials. (Griffin, S., Lane, S. A., and Leo, D., Power Consumption for Active Acoustic Control of Launch Vehicle Payload Fairings, 2000 International Mechanical Engineering Congress and Exposition, Orlando, Fla., November 2000.) Obviously, the device must be lightweight. A key motivation for using composite fairings is to reduce fairing weight and facilitate launching larger payload mass. Therefore, it would be impractical to develop acoustic treatments that significantly infringe on the weight savings. Robustness of the control approach must be assured. As each launch costs millions of dollars in addition to the millions of dollars that the payload costs, any acoustic treatment must involve zero risk.
Another critical aspect in mitigating the launch environment is the ability to adapt to the time-varying nature of the environment. During accent, the intensity and sources of the vibro-acoustic disturbances vary. Also, the acoustic wave-speed and air density change during accent, which results in significant variations of the acoustic resonance frequencies. Likewise, the structural resonance frequencies will also vary. A unique requirement for launch vehicle actuators is that they are subjected to vacuum pressure in space. Therefore, any actuator design must adequately consider the effects that this transition imposes on the device.
Prior art in acoustic mitigation has failed to adequately address the specific conditions experienced during launch. Passive approaches, such as taught in U.S. Pat. No. 6,170,605, provide little absorption at low frequencies where a significant amount of acoustic energy is concentrated due to the acoustic resonances of the fairing cavity. Active noise control strategies suggested for acoustic mitigation in aircraft and automobiles are not practical for launch vehicles. Methods taught in the prior art, such as U.S. Pat. No. 5,485,523, U.S. Pat. No. 5,590,849, or U.S. Pat. No. 5,778,081, include multiple sensors, actuators, or controllers, which must be networked together by wires to sense the disturbance, generate a control signal, and then apply the control signal to the actuators. This would be unacceptable in launch vehicle applications, due to the inability to arbitrarily place sensors, actuators, and wiring throughout the fairing.
There have been acoustic devices designed to dissipate acoustic energy in enclosures, such as those taught by U.S. Pat. No. 5,771,300, U.S. Pat. No. 5,848,169, U.S. Pat. No. 5,974,155, U.S. Pat. No. 6,151,396, and U.S. Pat. No. 6,138,947. Many of these inventions use feedback loops with motion or pressure sensors to servo-control a speaker diaphragm, and increase the resistive acoustic impedance of the device, hence dissipating acoustic energy. Some devices are used directly as acoustic actuators to produce anti-noise, which are sound waves generated out-of-phase to a primary disturbance source. This results in cancellation of the noise under optimal circumstances.
The prior art in acoustic attenuation does not incorporate a mechanism to increase the transmission loss through the fairing. Although a primary feature of the present invention is the ability to damp low frequency acoustic cavity resonances, significant benefit can be achieved by changing the structural impedance of the fairing wall and thereby reducing the transmissibility of external vibro-acoustic energy. Prior art does not consider the loss of pressure in the fairing during accent, or the variation of acoustic resonances due to changes in the acoustic wave speed. These problems are specific to launch vehicles.
The present invention is a stand-alone, adaptive vibro-acoustic damper device. Multiple devices would be attached to the inside of a launch vehicle fairing structure at discrete locations. The devices provide both acoustic and structural energy dissipation, and reactive structural impedance to reduce the transmission of external vibro-acoustic energy into the structure. The device provides an efficient method of coupling to the fairing acoustic dynamics and dissipating energy and is particularly suited for attenuating low frequency acoustic energy. It is vented to enable pressure equalization during accent. Adaptability of the device to changing environmental parameters is achieved using a servo-control loop.